"I heard NASA did some research on methods of torquing fasteners. Their conclusion was that the most accurate and reliable method they could find was a competent mechanic with a hand wrench." -3-8. Also, hand wrenches have a particular length for a reason -- they naturally provide an appropriate lever length. This is why every time I've over-tightened I find I'm using a too-large ratchet driver -- or torque wrench! Those things have huge lever arms.